Self-regulating heaters utilizing electrically semi-conductive compositions having a positive temperature coefficient of electrical resistance and containing restrictively prescribed amounts of electrically conductive particles, such as carbon black, are well known in the prior art.
Generally, a material which exhibits a positive temperature coefficient of electrical resistance is a material whose electrical resistance increases as a result of an increase in its temperature. It is believed by many that polymeric compositions containing dispersed electrically conductive particles, such as carbon black, exhibit a positive temperature coefficient of electrical resistance as a result of the polymeric matrix expanding at a rate greater than that of the electrically conductive particles when subjected to an increase in temperature. It has been theorized that such polymeric matrix expansion tends to increase, or otherwise alter, the spacial relationship between the electrically conductive particles in such a manner as to result in an increase in the electrical resistance of the polymeric composition. An increase in the electrical resistance of the polymeric composition would correspondingly reduce the amount of electrical current derived from a fixed electrical potential placed across the composition and reduce the amount of heat generated by the electrical current according to the established relationship of heat equals I.sup.2 R.
It is the theory of others that the amount of crystallinity present in a polymeric composition containing electrically conductive particles is an important factor in providing a useful positive temperature coefficient of electrical resistance. According to this train of thought, an increase in electrical resistance may arise as a result of the reorientation of the crystalline-amorphic boundaries when the polymeric composition's temperature is caused to increase and which, aside from whether or not the composition expands during its increase in temperature, tends to electrically insulate the conductive particles (or groups of the electrically conductive particles) more effectively from each other and thereby contributes to an increase in the all-over electrical resistance of the composition.
Previous studies of polymeric compositions containing varying amounts of dispersed electrically conductive carbon blacks have shown certain characteristics as to the magnitude of increase of electrical resistance per thermal unit of temperature increase. Such studies have also resulted in derived terminology that is useful in describing certain relationships. Generally, the type and make-up of the polymeric composition; the nature, physical size and amount of electrically conductive particles; and the method by which they are dispersed in the polymeric matrix determines the value of derived terms such as, for example, R.sub.25 (electrical resistance at 25.degree. C.); T.sub.c (controlling temperature about which the electrical resistance increases or decreases in response to an electrical current having a fixed potential; R.sub.p (peak electrical resistance above which the electrical resistance of the semi-conductive composition begins to reverse itself and decrease rapidly in response to an increase in temperature in association with the melt phase of the polymeric composition; and R.sub.p /R.sub.25 (the ratio of the above described electrical resistances generally depicting the range of resistance between the given two temperature points.
Until the time of the present invention, it was thought that in order to provide a useful electrically semi-conductive heating device the amount of electrically conductive carbon black particles dispersed in the polymeric composition must be either 15% or less or 25% or more, by weight, of the total weight of the composition. An example of such compositions can be found in Kohler's U.S. Pat. No. 3,243,573 wherein the electrically semi-conductive compositions are described as containing 25 to 75 percent by weight carbon black as a result of in-situ polymerization. Although such compositions may be useful for some heating purposes, it has been found that polymeric compositions containing more than 25% by weight of carbon black generally possess poor cold temperature properties; exhibit inferior elongation characteristics; and generally do not possess good electrical current regulating characteristics in response to changes in temperature. As noted above, it has also been proposed that electrically semi-conductive compositions must not have more than 15% by weight of carbon black in order to provide a useful self-regulating heating device. Such teaching can be found, for example, in U.S. Pat. No. 3,793,716 in which a process is described for making a self-regulating heating element utilizing a composition having less than 15% by weight of carbon black incorporated therein. This contention is also maintained in U.S. Pat. No. 3,861,029 wherein a polymeric material containing not more than about 15% by weight of carbon black is subjected to a prolonged annealing procedure to reduce its electrical volume resistivity at room temperature to from about 5 to about 100,000 ohm-cm.
A further extension of this belief can be found in U.S. Pat. No. 3,914,363 wherein a shape retaining thermoplastic jacket is disposed about self-regulating conductive articles utilizing crystalline polymeric compositions containing not more than about 15% by weight of conductive carbon black and the combination thereof is subjected to an annealing procedure whereby the room temperature electrical volume resistivity of the polymeric composition is reduced to within the range of from about 5 to about 100,000 ohm-cm. This contention is also reiterated in U.S. Pat. No. 3,823,216 wherein a cyclic annealing process is disclosed and claimed for reducing the electrical volume resistivity to a value within the range of from about 5 to about 100,000 ohm-cm at 70.degree. F. for compositions disclosed therein which are used in self-temperature regulating articles and which contain carbon lack dispersed therein in an amount not greater than about 15% by weight to the total weight of the composition.
Electrically conductive compositions can additionally be found, for example, in U.S. Pat. No. 2,750,482 in which is disclosed an amorphous polyisobutylene material containing conducting particles for use in high temperature alarms and in U.S. Pat No. 2,905,919 in which an electrical heating cable is described as containing a semi-conductive body of pulverulent inorganic material. A further example of an electrically semi-conductive composition can be found in U.S. Pat. No. 3,179,544 in which an electrically conductive article is produced by depositing an electrically conductive composition comprising an aqueous dispersion of graphite particles upon an insulating base. Still further examples of electrically semi-conductive compositions can be found in U.S. Pat. No. 2,803,566 in which an article is disclosed having a coating thereupon of a mixture of colloidal silica, substantially free of alkalai and in U.S. Pat. No. 3,413,442 in which a semi-conductive material is disclosed having a steep slooped positive temperature coefficient for use in electrical heating devices in the form of an open ended container.